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Department of Basic Science/ College of Dentistry/ University of Baghdad

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General Human Histology

Lecture 6 Assist. Prof. Ahmed Anwar Albir

Respiratory System

2- The Respiratory Portion of the Respiratory System

Respiratory Bronchiole

Each terminal bronchiole (Figure 14) subdivides into 2 or

more respiratory bronchiole that serve as regions of transition

between the conducting and respiratory portions of the respiratory

system (Figure 15). The respiratory bronchiolar mucosa is structurally

identical to that of the terminal bronchioles, except that their walls

are interrupted by numerous saclike alveoli where gas exchange

occurs. Portions of the respiratory bronchioles are lined with ciliated

cuboidal epithelial cells and Clara cells, but at the rim of the alveolar

openings the bronchiolar epithelium becomes continuous with the

squamous alveolar lining cells (type I alveolar cells). Proceeding

distally along these bronchioles, the alveoli increase greatly in

number, and the distance between them is markedly reduced. Between

alveoli, the bronchiolar epithelium consists of ciliated cuboidal

epithelium; however, the cilia may be absent in more distal portions.

Smooth muscle and elastic connective tissue lie beneath the epithelium

of respiratory bronchioles (Figures 15 and 16).


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Figure 15. Photomicrograph of a thick section of lung showing

a terminal bronchiole dividing into 2 respiratory bronchioles, in

which alveoli appear. The Spongelike appearance of the lung is due

to the abundance of alveoli and alveolar sacs.

Figure 16. Respiratory bronchiole.


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Alveolar Ducts

Proceeding distally along the respiratory bronchioles, the number

of alveolar openings into the bronchiolar wall becomes ever greater

until the wall consists of nothing else, and the tube is now called

an alveolar duct (Figure 17). Both the alveolar ducts and the alveoli

(Figures 13 and 18) are lined with extremely attenuated squamous

alveolar cells. In the lamina propria surrounding the rim of the alveoli

is a network of smooth muscle cells. These sphincterlike smooth

muscle bundles appear as knobs between adjacent alveoli. Smooth

muscle disappears at the distal ends of alveolar ducts. A rich matrix

of elastic and collagen fibers provides the only support of the duct and

its alveoli.

Alveolar ducts open into atria that communicate with alveolar

sacs, 2 or more of which a rise from each atrium. Elastic and reticular

fibers form a complex network encircling the openings of atria, alveolar

sacs, and alveoli. The elastic fibers enable the alveoli to expand with

inspiration and to contract passively with expiration. The reticular

fibers serve as a support that prevents overdistention and damage to

the delicate capillaries and thin alveolar septa.


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Figure 17. Diagram of a portion of the bronchial tree. Note that the

smooth muscle in the alveolar duct disappears in the alveoli.

Figure 18. Alveolar walls (interalveolar septa)


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Alveoli

Alveoli are saclike evaginations (about 200µm in diameter) of

the respiratory bronchioles, alveolar ducts, and alveolar sacs. Alveoli

are responsible for the spongy structure of the lungs (Figure 15).

Structurally, alveoli resemble small pockets that are open on one

side, similar to the honeycombs of a beehive. Within these cuplike

structures, O2 and CO2 are exchanged between the air and the blood.

The structure of the alveolar walls is specialized for enhancing

diffusion between the external and internal environments. Generally,

each wall lies between 2 neighboring alveoli and is therefore called

an interalveolar septum or wall. An interalveolar septum (Figures 19

through 20) consists of 2 thin squamous epithelial layers between

which lie capillaries, elastic and reticular fibers, and connective tissue

matrix and cells. The capillaries and connective tissue constitute

the interstitium.

Within the interstitium of the interalveolar septum is found

the richest capillary network in the body.


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Figure 19. Three-dimensional schematic diagram of pulmonary

alveoli showing the structure of the interalveolar septum. Note the

capillaries, connective tissue, and macrophages. These cells can

also be seen in-or passing into-the alveolar lumen. Alveolar pores

are numerous. Type II cells are identified by their abundant

apical microvilli. The alveoli are lined with a continuous epithelial

layer of type I cells.


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Figure 20. Section of a lung fixed by intra-alveolar injection of

fixative. Observe in the interalveolar septum 3-laminar structures

(arrowheads) constituted by a central basement membrane and

2 very thin cytoplasmic layers. These layers are formed by the

cytoplasm of epithelial cell type I and the cytoplasm of capillary

endothelial cells. High magnification.

Walls of alveoli are composed of two types of cells: type I

pneumocytes and type II pneumocytes.


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1. Type I Pneumocytes

They are extremely attenuated cells that line the alveolar surfaces.

Type I cells composed of simple squamous epithelium (also called type I

alveolar cells and squamous alveolar cells) make up 97% of the alveolar

surfaces.

These cells are so thin (sometimes only 25nm) that the electron

microscope was needed to prove that all alveoli are covered with

an epithelial lining (Figures 19 and 21). Organelles such as the Golgi

complex, endoplasmic reticulum, and mitochondria are grouped

around the nucleus, reducing the thickness of the blood-air barrier

and leaving large areas of cytoplasm virtually free of organelles.

The cytoplasm in the thin portion contains abundant pinocytotic

vesicles, which may play a role in the turnover of surfactant

(described below) and the removal of small particulate contaminants

from the outer surface. In addition to desmosomes, all type I epithelial

cells have occluding junctions that prevent the leakage of tissue

fluid into the alveolar air space (Figure 22). The main role of these

cells is to provide a barrier of minimal thickness that is readily

permeable to gases.


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Figure 21. Electron micrograph of the interalveolar septum. Note the

capillary lumen, alveolar spaces, alveolar type I epithelial cells, fused

basal laminae, and a fibroblast.

Figure 22. Cryofracture preparation showing an occluding junction

between 2 type I epithelial cells of the alveolar lining.


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2. Type II Pneumocytes

They make up the remaining 3% of the alveolar surfaces (also

known as great alveolar cells, septal cells, and type II alveolar cells).

Type II cells are interspersed among the type I alveolar cells with which

they have occluding and desmosomal junctions (Figures 23 and 24).

Type II cells are rounded cells that are usually found in groups of

2 or 3 along the alveolar surface at points where the alveolar walls

unite and form angles. These cells, which rest on the basement

membrane, are part of the epithelium, with the same origin as the

type I cells that line the alveolar walls. They divide by mitosis to

replace their own population and also the type I population. In

histologic sections, they exhibit a characteristic vesicular or foamy

cytoplasm. These vesicles are caused by the presence of lamellar

bodies (Figures 23 and 24) that are preserved and evident in tissue

prepared for electron microscopy. Lamellar bodies, which average

1-2µm in diameter, contain concentric or parallel lamellae limited

by a unit membrane. Histochemical studies show that these bodies,

which contain phospholipids, glycosaminoglycans, and proteins,

are continuously synthesized and released at the apical surface of

the cells. The lamellar bodies give rise to a material that spreads

over the alveolar surfaces, providing an extracellular alveolar

coating, pulmonary surfactant, that lowers alveolar surface tension.

Pulmonary surfactant, synthesized on the rough endoplasmic

reticulum (RER) of type II pneumocytes, is composed primarily

of two phospholipids, dipalmitoyl phosphatidylcholine and

phosphatidylglycerol, and four unique proteins, surfactant proteins

A, B, C, and D. The surfactant is modified in the Golgi apparatus

and is then released from the trans- Glogi network into secretory


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vesicles, known as composite bodies, the immediate precursors of

lamellar bodies. Pulmonary surfactant serves several major functions

in the economy of the lung, but it primarily aids in reducing the

surface tension of the alveolar cells. The reduction of surface tension

means that less inspiratory force is needed to inflate the alveoli, and

thus the work of breathing is reduced. In addition, without surfactant,

alveoli would tend to collapse during expiration. In fetal development,

surfactant appears in the last weeks of gestation and coincides with

the appearance of lamellar bodies in the type II cells.

The surfactant layer is not static but is constantly being turned

over. The lipoproteins are gradually removed from the surface by the

pinocytotic vesicles of the squamous epithelial cells, by macrophages,

and by type II alveolar cells.

Alveolar lining fluids are also removed via the conducting

passages as a result of ciliary activity. As the secretions pass up

through the airways, they combine with bronchial mucus, forming

a bronchoalveolar fluid, which aids in the removal of particulate

and noxious components from the inspired air. The bronchoalveolar

fluid contains several lytic enzymes (e g, lysozyme, collagenase,

β-glucuronidase) that are probably derived from the alveolar

macrophages.


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Figure 23. Secretion of surfactant by a type II cell. Surfactant

is a protein-lipid complex synthesized in the rough endoplasmic

reticulum and stored in the lamellar bodies. It is continuously

secreted by means of exocytosis (arrows) and forms an overlying

monomolecular film of lipid covering an underlying aqueous

hypophase. Occluding junctions around the margins of the

epithelial cells prevent leakage of tissue fluid into the alveolar

lumen.


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Figure 24. Electron micrograph of a type II cell protruding into the

alveolar lumen. Arrows indicate lamellar bodies containing newly

synthesized pulmonary surfactant. RER, rough endoplasmic reticulum;

G, Golgi complex; RF, reticular fibers. Note the microvilli of the type II

cell and the junctional complexes (JC) with the type I epithelial cell.

Blood- Air Barrier

Air in the alveoli is separated from capillary blood by 3

components referred to collectively as the blood-air barrier: the surface

lining and cytoplasm of the alveolar cells; the fused basal laminae of the

closely apposed alveolar and endothelial cells; and the cytoplasm of the

endothelial cells (Figure 25). The total thickness of these layers varies


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from 0.1 to 1.5µm. During inspiration, oxygen-containing air enters the

alveolar spaces of the lung. Because the total surface area of all the

alveoli exceeds 140m2 and the total blood volume in all of the capillaries

at any one time is no more than 140ml, the space available for diffusion

of gases is enormous. Oxygen diffuses through the blood-air barrier

to enter the lumina of the capillaries and binds to the heme portion

of the erythrocyte hemoglobin, forming oxyhemoglobin. CO2 leaves

the blood (liberation of CO2 from H2CO3 is catalyzed by the enzyme

carbonic anhydrase present in erythrocytes), diffuses through the

blood-air barrier into the lumina of the alveolus, and exits the alveolar

spaces as the CO2-rich air is exhaled. The passage of O2 and CO2 across

the blood- air barrier is due to passive diffusion in response to the partial

pressures of these gases within the blood and alveolar lumina.

Figure 25. Portion of the interalveolar septum showing the blood-air

barrier.


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Lung (Alveolar) Macrophages

Alveolar macrophages, also called dust cells, are found in the

interior of the interalveolar septum and are often seen on the surface

of the alveolus (Figure 20).

These cells phagocytose particulate matter, such as dust and

bacteria, and thus maintain a sterile environment within the lungs. These

cells also assist type II pneumocytes in the uptake of surfactant.

Approximately 100 million macrophages migrate to the bronchi each

day and are transported from there by ciliary action to the pharynx to be

eliminated by being swallowed or expectorated. Some alveolar

macrophages, however, reenter the pulmonary interstitium and migrate

into lymph vessels to exit the lungs.

Medical Application

In congestive heart failure, the lungs become congested with blood,

and erythrocytes pass into the alveoli, where they are phagocytized by

alveolar macrophages. In such cases, these macrophages are called heart

failure cells when present in the lung and sputum; they are identified by

a positive histochemial reaction for iron pigment (hemosiderin).

Alveolar Pores

The interalveolar septum contains pores, 10-15µm in diameter, that

connect neighboring alveoli (Figure 19). These pores equalize air

pressure in the alveoli and promote the collateral circulation of air when

a bronchiole is obstructed.


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Alveolar- Lining Regeneration

Inhalation of NO2 destroys most of the cells lining the alveoli

(type 1 and type II cells). The action of this compound or other toxic

substances with the same effect is followed by an increase in the mitotic

activity of the remaining type II cells. The normal turnover rate of type II

cells is estimated to be 1% per day and results in a continuous renewal of

both its own population and that of type I cells.

Medical Application

Emphysema is a chronic lung disease characterized by

enlargement of the air space distal to the bronchioles, with destruction

of the interalveolar wall. Emphysema usually develops gradually and

results in respiratory insufficiency. The major cause of emphysema is

cigarette smoking. Even moderate emphysema is rare in nonsmokers.

Probably irritation produced by cigarette smoking stimulates the

destruction, or impairs the synthesis, of elastic fibers and other

components of the interalveolar septum.

Gross Structure of the Lungs

The left lung is subdivided into two lobes and the right lung into

three lobes. Each lung has a medial indentation, the hilus, where the

primary bronchi, bronchiolar arteries, and pulmonary arteries enter and

the bronchiolar veins, pulmonary veins, and lymph vessels leave the lung.

This group of vessels and the airway that enter the hilus make up the root

of the lung.


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Each lobe is subdivided into several bronchopulmonary segments

supplies by a tertiary intrapulmonary (segmental) bronchus. In turn,

bronchopulmonary segments are subdivided into many lobules, each

served by a bronchiole. Lobules are separated from one another by

connective tissue septa, in which lymph vessels and tributaries of

pulmonary veins travel. Branches of bronchial and pulmonary arteries

follow bronchioles in their passage through the center of the lobule.

Pulmonary Blood Vessels

Circulation in the lungs includes both nutrient (systemic) and

functional (pulmonary) vessels. Pulmonary arteries and veins represent

the functional circulation. Pulmonary arteries are thin-walled as a result

of the low pressures (25 mm Hg systolic, 5 mm Hg diastolic) encountered

in the pulmonary circuit. Within the lung the pulmonary artery

branches, accompanying the bronchial tree (Figure 26). Its branches are

surrounded by adventitia of the bronchi and bronchioles. At the level

of the alveolar duct, the branches of this artery form a capillary network

in the interalveolar septum and in close contact with the alveolar

epithelium. The lung has the best-developed capillary network in the

body, with capillaries between all alveoli, including those in the

respiratory bronchioles.

Venules that originate in the capillary network are found singly in

the parenchyma, somewhat removed from the airways; they are supported

by a thin covering of connective tissue and enter the interlobular septum

(Figure 26). After veins leave a lobule, they follow the bronchial tree

toward the hilum.


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Nutrient vessels follow the bronchial tree and distribute blood to

most of the lung up to the respiratory bronchioles, at which point they

anastomose with small branches of the pulmonary artery.

Figure 26. Blood and lymph circulation in a pulmonary lobule.

Both vessels and bronchi are enlarged out of proportion in this

drawing. In the interlobular septum, only one vein (on the left) and

one lymphatic vessel (on the right) are shown, although both actually

coexist in both regions. At the lower left, an enlargement of the

pleura shows its mesothelial lining.


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Pulmonary Lymphatic Vessels

The Lymphatic vessels (Figure 26) follow the bronchi and

the pulmonary vessels; they are also found in the interlobular

septum, and they all drain into lymph nodes in the region of the

hilum. This lymphatic network is called the deep network to

distinguish it from the superficial network, which includes the

lymphatic vessels in the visceral pleura. The lymphatic vessels of the

superficial network drain toward the hilum. They either follow the

entire length of the pleura or penetrate the lung tissue via the interlobular

septum.

Lymphatic vessels are not found in the terminal portions of the

bronchial tree or beyond the alveolar ducts.

Nerves

Both parasympathetic and sympathetic efferent fibers innervate

the lungs; general visceral afferent fibers, carrying poorly localized

pain sensations, are also present. Most of the nerves are found in the

connective tissues surrounding the larger airways.

Pleura

The pleura (Figure 26) is the serous membrane covering the

lung. It consists of 2 layers, parietal and visceral, that are continuous in

the region of the hilum. Both membranes are composed of mesothelial

cells resting on a fine connective tissue layer that contains collagen and


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elastic fibers. The elastic fibers of the visceral pleura are continuous with

those of the pulmonary parenchyma.

The parietal and visceral layers define a cavity entirely lined

with squamous mesothelial cells. Under normal conditions, this pleural

cavity contains only a film of liquid that acts as a lubricant, facilitating

the smooth sliding of one surface over the other during respiratory

movements.

In certain pathologic states, the pleural cavity can become

a real cavity, containing liquid or air. The walls of the pleural cavity,

like all serosal cavities (peritoneal and pericardial), are quite

permeable to water and other substances-hence the high frequency of

fluid accumulation (pleural effusion) in this cavity in pathologic

conditions. This fluid is derived from the blood plasma by exudation.

Conversely, under certain conditions, liquids or gases in the pleural

cavity can be rapidly absorbed.

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